Heat exchangers are per se known in the art and serve to exchange heat between a heat dissipating medium which is to be cooled (hereinafter referred to as primary fluid) and a heat absorbing medium which is to be heated (hereinafter referred to as secondary fluid or “cooling fluid”), the two mediums, in heat exchangers of the type relevant in the present context, being spatially separated from each other, in particular in such a way that they do not mix. Such heat exchangers are used in connection with various industrial applications such as, for example, in a cooler as used for the cooling of gas turbine cooling air, in particular in a gas turbine power plant or a gas and steam turbine power plant, for the purpose of cooling air heated by compression (the primary fluid) and thus to improve the cooling effect in the gas turbine. Another application refers to process gas coolers in the chemical industry, for example syngas coolers in ammonia plants. Besides cooling air to be cooled, which often has temperatures of more than 400° C. and sometimes even more than 450° C. when entering the cooler for gas turbine cooling air, the secondary fluid is frequently water.
The heat exchangers used in these fields can be divided into different groups with respect to their design. One group is comprised of the so-called tube bundle heat exchangers, the basic design of which is also already known in the art. Tube bundle heat exchangers are characterized in that they comprise a plurality of tubes which are typically arranged so as to extend in parallel and in which, depending on the respective application, the primary fluid (for example air) flows in a direction of flow. At the same time, the secondary fluid flows around the outside surface of the tubes such that a heat exchange can occur between the two fluids.
Such heat exchangers further frequently comprise a pressure vessel, in particular in the shape of, for example, a hollow cylinder. The pressure vessel for example comprises an essentially hollow-cylindrical outer casing which is delimited at its face sides by an inlet tubesheet and an outlet tubesheet, which can more particularly be welded to the outer casing. A tubesheet is normally a disc-like component which carries the tubes forming the tube bundle. The term ‘inlet tubesheet’ designates that tubesheet through which the primary fluid passes when entering the pressure vessel on the tube side. The term ‘outlet tubesheet’ designates that tubesheet which is located downstream as regarded in the direction of flow of the primary fluid and through which the primary fluid passes when leaving the pressure vessel. The outer casing, the inlet tubesheet and the outlet tubesheet thus together enclose an interior space of the pressure vessel, the tubes forming the tube bundle passing through said interior space. Accordingly, the front and rear end sections of the tubes are accommodated by respective openings in the tubesheets. If the tubes are arranged so as to extend in straight lines in the longitudinal direction such that the longitudinal axis of the pressure vessel runs parallel to the longitudinal axis of the tubes, the tube bundle heat exchanger has a straight-tube configuration. This type of tube bundle heat exchangers is characterized in particular by the individual tubes running through the pressure vessel in a completely straight-lined fashion. They are thus not bent, for example, so as to follow a U-shaped pattern. The inlet tubesheet and the outlet tubesheet are consequently arranged at a distance from and opposite each other. During operation of the tube bundle heat exchanger the secondary fluid (in particular cooling water) flows through the hollow cylinder, i.e., the pressure vessel, along the outside surface of the tubes, while the primary fluid (in particular air) flows through the pressure vessel inside the tubes. This process causes the warm fluid to cool off and, due to the heat exchange, the cool fluid to be heated. For example, for purposes of dissipating heat from gas turbine cooling air, water will typically flow around the tubes, that is, around the outside surfaces of the tubes, on the casing side inside the pressure vessel. This can be done without a phase change. However, this process frequently causes the water to vaporize, resulting in an increased energy consumption due to the high vaporization enthalpy. The mass flow is thus kept small on the casing side. The generated steam can be discharged into a waste heat boiler or be used as process steam. Thus, as an alternative or a complement to the cooling fluid outlet (outlet for liquid cooling fluid), the pressure vessel may also comprise a steam outlet (outlet for vaporized cooling fluid) adjacent to the cooling fluid inlet, in particular in the case of the cooling fluid being water. The primary fluid is usually fed into the tubes via an air chamber comprising an air inlet and thence through the inlet tubesheet. A plurality of tubes of the tube bundle heat exchanger, and in particular all of them, opens into the air chamber. After passing through the tubes of the tube bundle heat exchanger, the air to be cooled leaves the tubes via the outlet tubesheet, gathering in an air chamber located downstream, from where it is then dissipated via an air outlet.
A problem found in prior art tube bundle heat exchangers is the fact that, with the tubes, more particularly their front and rear ends, normally being tightly rolled into the tubesheets, the latter suffer high thermal stress during operation. Especially due to the tubesheets typically being fixed to the casing of the hollow cylinder and high temperature gradients occurring between the cold side and the warm side of the heat exchanger in many applications such as, for example, in the case of boilers, cogeneration etc., high thermal loads act on the tubesheets, especially on the high temperature side, i.e., the side of the inlet tubesheet, where the primary fluid is fed into the straight tubes. A particular problem is the uneven heat distribution between the tubes and the tubesheet in the radial direction of the tubesheet. This may cause damage to the tubesheet material, so that the tube bundle heat exchanger may lose its operability. Further, as regards the design of a generic tube bundle heat exchanger, in particular for use in a cooler for gas turbine cooling air, account must be taken of the indispensability of stainless materials due to high cleanliness requirements with respect to the tube side of the gas turbine, i.e., the inside surface of the tubes. At the same time, however, it is necessary to sufficiently address the occurrence of high temperatures as caused by the primary fluid to be cooled, which can reach temperatures of sometimes more than 450° C., in particular in the case of gas turbine cooling air. Accordingly, the inlet section towards the inlet tubesheet as well as the tubes in this section should be resistant to temperatures of approx. 500° C. and up to550° C. For reasons of cost efficiency, the pressure vessel is normally made of an unalloyed pressure vessel steel of the type specified in particular in the DIN EN (Deutsches Institut fuer Normung—German Institute for Standardization; Europaeische Norm—European standard) 10028 family of standards. During operation, the cooling air to be cooled heats the components located in the inlet section of the tube bundle heat exchanger, such as for example the chamber upstream of the inlet tubesheet as well as the tubesheet itself, relatively to the casing of the pressure vessel, to a considerable extent. This results in temperature differences of up to 200 K in this area, which in turn cause high stress and deformation. For this reason, a so-called casing compensator is necessary for the purpose of compensating for the temperature-induced changes in length, in particular in the case of straight-tube configurations facing considerable temperature differences between the tube side and the casing side and/or in the case of material combinations comprising austenitic heat exchanger tubes and a ferritic casing with deviating coefficients of expansion. Such casing compensators are disadvantageous in that they are relatively expensive and require high maintenance. Among other things, their welding seams and moving parts frequently make them an additional weak point and a source of leakage. Moreover, friction forces from the saddles of the floating bearings as well as tube forces cannot be transferred by the casing but only by the heat exchanger tubes. This means heavy loads acting on the elements connecting the tube with the tubesheet. In addition, customers frequently demand systems, for example coolers for gas turbine cooling air, which dispense with such casing compensators.
Furthermore, when chloride containing mediums are used on the casing side, some tube materials are apt to suffer so-called stress corrosion cracking (SCC). This phenomenon predominantly occurs at the connections between the tubes and the tube sheet, where chlorides may collect in the gaps. The stress forces required for this result from thermal expansion and deformation of the structure. SCC may cause damage very rapidly, even within days. The austenitic tube material that is frequently used for the tubes in generic heat exchanges having a straight-tube configuration (in particular 1.4301 or 1.4306 in accordance with DIN EN 10088-1:2005, or TP304 or TP304L in accordance with ASTM or ASME) often lacks sufficient resistance to SCC. Water is vaporized on the casing side of the pressure vessel, resulting in an undesired concentration of chlorides in this area. Said chlorides collect in the gap between the tube and the tubesheet. Failure of a cooler for cooling air can shut down a complete gas power plant as long as it is under repair.